1. Field of the Invention
This invention relates to improvement of loudspeaker systems and particularly to such selection and arrangement of each of loudspeakers on an enclosure of a loudspeaker system and determination of the reproduction frequency ranges covered by the respective loudspeaker as to provide natural and life-like sounds reproduction substantially independent of the acoustic conditions of the listening room.
2. Description of the Prior Art
It is desired that a loudspeaker system should provide the listener with sounds reproduced at a same level independent of the frequency.
In this regard, in conventional practice the contribution of indirect sounds effect has been disregarded and too much importance has been attached to the frequency response characteristics which are normally determined by measuring the sound pressure in front of the loudspeaker in an anechoic room, altering the frequency of the sounds. In other words, importance has been attached to only the characteristics of the sounds which reaches the listener directly from the loudspeakers.
Thus the main object of the conventional designing has been simply to flatten or level off the frequency response characteristics over a sufficiently wide frequency range.
For example, taking a conventional 2-way loudspeaker system as shown in FIG. 1, there is provided a low range loudspeaker (1) and a high range loudspeaker (2) on an enclosure (3).
Shown in FIG. 2 is a combined chart of frequency response characteristics of the loudspeaker system of FIG. 1, as measured in an anechoic room, where curves (A) (B) (C) represent those of the low range loudspeaker (1), the high range loudspeaker (2) and the total system of the combination of both of them, respectively. On the other hand, shown in FIG. 3 is a combined chart of the corresponding sound power characteristics, as measured in an reverberant room, where curves (A') (B') (C') correspond to (A) (B) (C) in FIG. 2, respectively. In designing such loudspeaker system, it is the objecting general way to aim at realizing flatness of the frequency response characteristics of the total system as far as possible, and for this purpose the network circuit is designed in such manner that input signal level to the respective loudspeaker shows 3dB attenuation at the crossover frequency f.sub.c, which is the frequency where said respective frequency response characteristics curves (A) (B) of said two loudspeakers cross with each other.
It should be noted here, however, that said frequency response characteristics are those of only the direct sounds on the radiation axis in an anechoic room, while in actual use in usual listening rooms the listener will hear not only the said direct sounds but a succession of indirect sounds reflected from the ceiling, side walls and the like, as well.
It is, therefore, desirable to design loudspeaker system in such manner that the sounds pressure as a sum of the direct and indirect sounds remains constant independently of the frequency. As is well known, however, loudspeaker have the directivity that will become sharper as reproduction sounds frequency becomes higher, which means that lower level of sounds pressure is radiated in directions away from the radiation axis, and thus weaker sounds pressure is reflected from the ceiling, side walls and the like, as the frequency becomes higher. Namely, the higher the reproduction sound frequency, the weaker will become the intensity of the indirect sounds.
When the sound power (defined by P=.intg.P.theta..phi.d.omega.: with P: sound power, i.e. sound source power output; .omega.: solid angle; P .theta..phi.: power intensity in the directional angles .theta., .phi. and approximated by P=.SIGMA.P.theta..phi..DELTA..omega. with P.theta..phi. measured in an anechoic room) is considered with the loudspeaker system of conventional design as mentioned at the beginning, the sound power level begins to decline, with respect to each of the low range loudspeaker (1) and the high range loudspeaker (2), at the respective particular frequency at which the directivity begins to become evident with respect to said each loudspeaker. A loudspeaker having by nature such characteristics, the conventional loudspeaker systems as described above give forth indirect sound pressure which varies depending upon the frequency, thus failing to provide life-like sound reproduction.
However, the role of indirect sounds has recently come to be taken into account in order that the listeners may hear life-like reproduced sounds, since the listener in fact hears simultaneously not only the direct sounds from the loudspeakers but also the indirect sounds reflected from the ceiling, walls and the like of the listening room. It is true that there have been already made some invention, such as U.S. Pat. Nos. 4,006,311 and 4,179,585, which provide positively indirect sounds, but these patents aim to widen the space of propagation of the reproduced sounds and to provide the indirect sounds in the high frequency respectively, rather than to level off the characteristics of the total sounds, and therefore the listener can not as yet hear life-like and natural sounds as expected.
Japanese Patent Publication Sho. 54-33854 discloses another type of invention whose object is to provide the listener with natural and life-like reproduced sounds in such a manner that sound power characteristics of the entire listening space as intended, is to be flat, under due consideration of the indirect sounds effect, but this type of loudspeaker system after all provided no other than the flat sound power characteristics, but did not provide the flat frequency response characteristics straight in front of the loudspeaker system, producing direct sounds which have no flat characteristics, which makes the listener feel unnatural in the sound reproduction.
It should be further noticed in this regard that it has heretofore been considered that the frequency f.sub.1 beyond which the sound power depression becomes evident on account of the directivity of the loudspeaker is a value derived theoretically in modelling the same as a piston, namely EQU f.sub.1 =c/2.pi.a
with c: sound velosity PA1 and a: effective vibration radius of the loudspeaker, PA1 with c: sound velocity PA1 and a: effective vibration radius of the respective loudspeaker,
but various experiments have now revealed that such frequency f.sub.1 is not quite true with respect to actual loudspeakers, thus making it clear that improvement is required also in this regard.
Consequently, it has been revealed that this is also one of the reasons why the conventional loudspeaker systems with the loudspeakers disposed on the front panel fail to provide flat sound power characteristics, as shown at (C) in FIG. 3, which consequently leads to unfavorable unnatural aural result on account of the uneven depression appearing in a particular frequency range in the indirect sounds.